In recent years, a compact image pickup device using a solid-state image sensor, such as a CCD (Charge Coupled Device) type image sensor or a CMOS (Complementary Metal Oxide Semiconductor) type image sensor, has been mounted on a mobile terminal, such as a portable telephone or a PDA (Personal Digital Assistant), and furthermore also on a notebook computer and the like, allowing not only audio information but image information to be transmitted to and from a remote place.
In the solid-state image sensor used for such an image pickup device, in recent years, the miniaturization of a pixel size progresses, and the increase in the pixel density and the miniaturization of the image sensor have been achieved. Furthermore, it also becomes possible to curve an image pickup surface and thus there is a need for a compact and high-performance image pickup lens optimum for such an image sensor.
Patent Literature 1 discloses an image pickup device with a curved solid-state image sensor. In Patent Literature 1, by curving the solid-state image sensor into a polynomial surface shape, the field curvature and distortion aberration that are caused by lenses are corrected in a balanced manner and a compact and high-resolution image pickup device is provided. However, because the solid-state image sensor has a CIF (Common Intermediate Format) size (352 pixels×288 pixels) and the image pickup lens has only one lens, chromatic aberration is not fully corrected. Therefore, a high-performance image pickup device cannot be expected to be obtained using a higher pixel density solid-state image sensor.
Patent Literature 2 and Patent Literature 3 disclose image pickup lenses having a curved image pickup surface, a photographing angle of approximately 77°, and the speed or brightness of F 5.7 to F 6.2 for the applications of a compact camera and a film unit with a lens. Here, the image pickup lens is a post-positioned aperture triplet type lens comprising a positive first lens, a negative second lens, a positive third lens, and an aperture stop.
However, in the case of the image pickup lenses of Patent Literature 2 and the like, a sufficient performance cannot be obtained with an F-number slower or darker than F 5, and the back focus of the triplet type lens as described above tends to be long, and therefore the sizes of the image pickup lens and the image pickup device will increase.
Moreover, Patent Literatures 2 and 3 relate to image pickup lenses for film cameras and attempt to improve performances by curving a film surface (image pickup surface) in accordance with the field curvature that is caused by the lens. However, each of the image pickup lenses is an image pickup lens for cameras using a roll film and therefore because of the structure of the camera, the film surface results in the so-called cylindrical image pickup surface where the film surface curves only in a long side direction of a picture plane. For this reason, while a favorable performance can be obtained in the long side direction of the picture plane, the image pickup surface in the short side direction of the picture plane remains planar, and therefore not only an improvement in the performance cannot be achieved but also degradation may be caused in accordance with the correction state of the field curvature. With the curving only in the long side direction of an image pickup surface as with Patent Literatures 2 and 3, it is difficult to obtain high performance across the entire picture plane. Therefore, usually the focal depth is set deep by setting the F-number of the lens slow so that a blur in the plane direction is prevented from becoming conspicuous, and therefore it has been difficult to set the F-number faster or brighter.
Furthermore, because the image pickup lenses of Patent Literatures 2 and 3 are the ones for film cameras as described above, incidence angles of principal rays of light rays incident upon an image pickup surface is not necessarily designed so as to be sufficiently small on the periphery of the image pickup surface. In the image pickup lens for forming an object image onto a photoelectric conversion unit of a solid-state image sensor, when principal ray incidence angles of light rays incident upon the image pickup surface, i.e., the so-called telecentric characteristic, degrades, the light rays will enter obliquely to the solid-state image sensor, and a phenomenon (shading) occurs which decreases an effective aperture efficiency on the periphery of the image pickup surface, resulting in an insufficient quantity of light in the periphery.
Patent Literature 4 discloses an example of achieving higher performance by combining four lenses with a curved solid-state image sensor. However, because a lens nearest to the image side is a positive lens, a total length is long and the aberration correction is also insufficient as compared with the total length, and thus these examples cannot meet with the higher pixel density and larger aperture in recent years.
On the other hand, as a compact and high-performance lens, an image pickup lens comprising five lenses has been proposed because it can achieve higher performance as compared with an image pickup lens comprising three or four lenses.
Patent Literature 5 discloses the so-called telephoto type image pickup lens comprising, in order from an object side, a first lens having a positive refractive power, a second lens having a negative refractive power, a third lens having a positive refractive power, a fourth lens having a positive refractive power, and a fifth lens having a negative refractive power and attempting to reduce a total length of an image pickup lens (a distance on an optical axis from a lens surface on a side closest to an object of an entire system of an image pickup lens to an image side focal point).
However, with regard to the image pickup lens described in Patent Literature 5, principal ray incidence angles with respect to an image pickup surface is 26° or more at the maximum and the telecentric characteristic is not good, and furthermore a reduction of the total optical length is also insufficient and the F-number is also as slow as approximately F 3.0.